Add agent skill

[virgesmith]

Ports virgesmith/xenoform-rs#24 to this repo: an agent skill bundled inside the package, plus a console script to install it into a project. Documented on the docs site rather than in the README.

What

• neworder/skill/SKILL.md — a compact reference for AI coding agents covering the modify → step/check → finalise lifecycle, the four timeline types (and halt()), the MonteCarlo and SplitMix64 engines with their seeding strategies, neworder.df.transition/transition_conditional/unique_index, spatial domains, MPI patterns, and the framework's recurring pitfalls (uninitialised base class, strict C++ typing, NEVER being NaN, per-agent python loops).
• neworder/skill_cli.py — a neworder-skill console script with --install [PATH] / --remove [PATH] (default PATH=.agents, so .agents/skills/neworder), registered under [project.scripts]. package_data in setup.py ships the skill in the wheel and sdist.
• docs/agent-skill.md + nav entry in zensical.toml, and a two-line pointer to it from README.md.
• test/test_skill_cli.py — 13 tests covering install/remove, default path, idempotency, the copy fallback and stale-copy refresh, and the refusals.

Notes

• The skill points at the documentation site rather than restating it. Its header table links to the overview, tips, examples and developer pages at neworder.readthedocs.io/en/stable/; the body is a summary an agent can hold in context, not a second copy of the docs. That is also why the user-facing docs for this feature are a docs-site page — the README is inlined into docs/index.md via include_snippet, so anything longer there would duplicate the new page on the site's front page.
• Symlink where possible, copy where not. A symlink to the bundled directory always matches the installed version, but CI and the classifiers cover Windows, where symlink_to needs developer mode — so --install falls back to shutil.copytree, and re-running it refreshes a copy (which, unlike a symlink, goes stale on upgrade).
• Nothing unrecognised is overwritten or deleted. A symlink is "ours" if it resolves to the bundled directory; a directory only if every entry is a file we ship. Anything else is left untouched and the command exits 1.
• Verified against a built wheel and sdist that neworder/skill/SKILL.md is packaged and the entry point registered — [tool.cibuildwheel] runs the test suite against the installed wheel, so the new tests would fail there otherwise.

Gates: ruff check, ruff format --check, ty check neworder examples test, pytest (102 passed, 1 skipped) all green; zensical build clean; examples/hello_world/model.py runs. JOURNAL.md entry added.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.57%. Comparing base (a2ec234) to head (a1dee91).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #122   +/-   ##
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  Coverage   95.57%   95.57%           
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  Files          15       15           
  Lines         723      723           
=======================================
  Hits          691      691           
  Misses         32       32           

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